This invention relates generally to bore-hole and well mapping, and more particularly concerns method and apparatus to remotely determine the azimuthal direction of a probe, which may for example be inserted into a bore-hole or well. In addition, it concerns method and apparatus to determine the probe's degree of tilt from vertical and to relate the latter to gyroscope generated azimuth information, at all latitudes and at all instrument attitudes. Further, the azimuth determining apparatus by itself or in combination with the tilt measuring apparatus, may be housed in a carrier of sufficiently small diameter to permit insertion directly into available small I.D. drill tubing, thus eliminating the need to remove the tubing to enable such mapping.
In the past, the task of position mapping a well or bore-hole for azimuth in addition to tilt has been excessively complicated, very expensive, and often inaccurate because of the difficulty in accommodating the size and special requirements of the available instrumentation. For example, magnetic compass devices typically require that the drill tubing be pulled from the hole and fitted with a length of non-magnetic tubing close to the drill head; or, the drill stem may be fitted with a few tubular sections of non-magnetic material, either initially or when drill bits are changed. The magnetic compass device is inserted within this non-magnetic section and the entire drill stem reassembled and run back in the hole as measurement are made. Thereafter, the magnetic compass instrumentation package must again be removed, requiring another round trip of the drill string. These devices are very inaccurate where drilling goes through magnetic materials, and are unusable where casing has been installed.
Directional or free gyroscopes are deployed much as the magnetic compass devices and function by attempting to remember a pre-set direction in space as they are run in the hole. Their ability to remember degrades with time and environmental exposure. Also, their accuracy is reduced as instrument size is reduced, as for example becomes necessary for small well bores. Further, the range of tilt and azimuthal variations over which they can be used is restricted by gimbal freedom which must be limited to prevent gimbal lock and consequent gyro tumbling.
A major advance toward overcoming these problems is described in my U.S. Pat. No. 3,753,296. That invention provides a method and means for overcoming the above complications, problems, and limitations by employing that kind and principal of a gyroscope known as rate-of-turn gyroscope, or commonly `a rate gyro`, to remotely determine a plane containing the earth's spin axis (azimuth) while inserted in a bore hole or well. The rate gyroscope has a rotor defining a spin axis; and means to support the gyroscope for travel in a bore-hole and to rotate about another axis extending in the direction of the hole, the gyroscope characterized as producing an output which varies as a function of azimuth orientation of the gyroscope relative to the earth's spin axis. Such means typically includes a carrier containing the gyroscope and a motor, the carrier being sized for travel in the well, as for example within the drill tubing. Also, circuitry is operatively connected with the motor and carrier to produce an output signal indicating azimuthal orientation of the rotating gyroscope relative to the carrier, whereby that signal and the gyroscope output may be processed to determine azimuth orientation of the carrier and any other instrument therein relative to the earth's spin axis, such instrument for example comprising a well logging device such as a radiometer, inclinometer, etc.
While highly accurate azimuth information is obtainable from the device and method of U.S. Pat. No. 3,753,296, certain problems can present themselves depending upon the bore-hole direction relative to the earth's spin axis.
Consider for example the case of a vertical bore-hole at or near the North pole. Since the travel axis of the instrument or navigator is parallel to the earth's spin axis, the navigator gimbal would rotate and its gyro's input axis would remain in a plane perpendicular to ESA (Earth Spin Axis) and would not detect earth's rate of rotation. Likewise, the accelerometer would have a zero output signal since its sensitive axis remains at right angles to the direction of gravity. This unique case at or near the earth's poles is repeated at lower latitudes, where for example the bore-hole is slanted to approach a parallel relation with ESA. As in the earlier example, the gyro's input axis is rotated in a plane perpendicular to earth's spin vector, and thus there is no output resulting from gyroscopic forces. An instrument mass unbalance error, or any uncertainty of .+-. one degree of rotation per hour, under these circumstances can not be differentiated from a bore-hole or instrument travel axis misalignment with ESA of about 4 degrees. In addition, since there is so little signal (due to earth's rotation) being detected, such uncertainties amplify their effects in the ability to define North. In applicant's standard scheme of using gyro-accelerometer signals for instance, such a .+-.1.degree./hour uncertainty would result in at least a .+-.50.degree. to 60.degree. North error at 5 degrees inclination from ESA. With a 10.degree. inclination from ESA, a .+-.1.degree./hour uncertainty would result in a .+-.20.degree. error in North.